The present invention generally relates to monitoring devices and more particularly relates to a method and a device for monitoring the motion of an actuator of a vehicle brake.
DE 197 14 046 A1 discloses an electromechanically operated wheel brake wherein an electric motor drives a rotating spindle. By way of a translatory movement of the spindle induced thereby, the brake is actuated by displacement of the brake linings against the brake disc or brake drum. In this arrangement, the spindle can move only within a fixed mechanical range.
This range results from the constructive design of brake mechanics which is normally rated so that the brake linings can be set or readjusted within a range of readjustment corresponding to lining wear in their initial position (release position). The setting is effected so that the brake linings are at a given distance from, i.e., have a clearance to, the brake disc or brake drum. In the other direction, the mechanical range is rated so that lining replacement is easily possible when needed (i.e. the spindle can be withdrawn to a sufficient extent in the direction of release of the brake).
To prevent the spindle from moving into abutment on the mechanical stop during operation and, on the other hand, to recognize the necessity of lining replacement in due time, the relative drive or spindle movement can be monitored.
An object of the present invention is to provide a method and a device for monitoring the motion of an actuator of a vehicle brake by which monitoring with respect to an absolute vehicle position is possible.
To monitor the spindle movement, magnets can be fitted to the driving engine shaft in an evenly spread fashion which produce pulses when passing a Hall sensor fitted at an appropriate location. These pulses are used to determine e.g. the course angle of the engine shaft relative to a previously defined reference position. From this value, in turn, the translatory movement of the spindle can be inferred by way of the transmission ratio between spindle rotation and spindle translation. The reference position for this position measuring system which performs relative and incremental measurement is fixed by zero point initialization, and changes of the course angle compared to this reference position are reviewed by counting the pulses produced by the position measuring system. Usually, the position of the drive shaft or of the spindle serves as a reference position, during which the brake linings are in straight (i.e., force-free) abutment on the brake disc or brake drum. The direction of motion of the drive can be sensed in addition by an appropriately installed second Hall sensor. In this case, it applies in general to the counting direction that a mathematically positive angular variation implies a movement of the driving shaft in the direction of brake application, and a negative angular variation means a movement in the direction of release of the brake.
However, the above system is a position measuring system with relative measurement, which determines spindle positions only relatively, without knowing which absolute position within the limited mechanical motion range the spindle presently adopts.
Therefore, the following steps, the sequence of which is not determined herein, will be performed according to the present invention: establishing an actuator reference coordinate associated with a predetermined actuator reference position, determining an actuator actual coordinate in accordance with the actuator reference coordinate and in accordance with the motion of the actuator, establishing a first permitted actuator motion range based on a permitted release end coordinate of the actuator for release of the brake and a first permitted brake application end coordinate of the actuator for application of the brake in accordance with the actuator reference coordinate and issuing of a signal, when the actuator actual coordinate is outside the first permitted actuator motion range.
The term xe2x80x98actuatorxe2x80x99 may refer to a moving part for actuation of the brake. The moving part may e.g. be a motor shaft or a spindle. The actuator is driven preferably electrically. The actuator position can be sensed rotatorily and translatorily, however, it is preferred that it is sensed rotatorily e.g. by means of a Hall sensor or a resolver. The given actuator reference position is a fixed position which the actuator may assume in a defined manner. That means, this position is an absolute position within the vehicle, preferably, within the vehicle brake and can, thus, be always reassumed.
A switching sensor element such as a microswitch or a switching Hall sensor, which can sense the translatory position of the spindle, can be used to find the actuator reference position. When the actuator reaches the actuator reference position predetermined by the sensor element, it will issue a switching pulse or switch from the logical condition xe2x80x9c0xe2x80x9d into the logical condition xe2x80x9c1xe2x80x9d, for example. To adopt this position, the actuator preferably moves at a low speed in the direction of release of the brake until the sensor element signals that the position is reached, e.g. by the switching pulse.
Another possibility of finding the actuator reference position, without the need of fitting an additional sensor element in the brake, may include predefining the mechanical stop of the actuator on the engine housing, preferably in the direction of release of the brake, as actuator reference position. To adopt this position, the actuator may preferably move at a very low speed in the direction of release of the brake. The engine current that drives the actuator, for example, will be sensed during this movement. When the actuator moves against the engine housing, the engine current will rise significantly because the engine operates in opposition to a system with a very high amount of rigidity in this case. This current rise permits detecting that the actuator reference position is reached and terminating the movement of the actuator.
A related actuator reference coordinate can be established at a given actuator reference position. This can be done in that allocated to a counter, which is also used to determine the actuator actual coordinate, is a value which can be stored in a memory, or input from externally and stored subsequently. This value may equal zero so that the counter is e.g. simply reset or initialized again. Another possibility of establishing the actuator reference coordinate includes storing the value which is in the counter when the actuator reference position is reached as actuator reference coordinate. Attention should be paid, if necessary, when the actuator reference coordinate is established that, depending on the status of the actuator reference position, the counter starting from the reference coordinate is in a position to count up and down and produces utilizable values.
Further, a first permitted actuator motion range can be established within which the actuator is allowed to move. This permitted motion range can refer to the position of the actuator or to the related coordinate because one can be determined by the respectively other one. This first actuator motion range is confined by two permitted coordinates, that is, by a permitted release end coordinate in the direction of release of the brake, and by a first permitted brake application end coordinate in the direction of application of the brake, the respective end coordinates pertaining to corresponding actuator end positions. The first permitted actuator motion range is established in accordance with the actuator reference coordinate and, thus, also in accordance with the actuator reference position. Thus, a motion range is established which has an absolute reference to the actuator reference position.
The above-described method steps can be performed in a service brake in each case when a vehicle is started. The corresponding coordinates can be stored and polled again at any time. In a parking brake, these steps can be performed when the brake is released which must be possible also during travel of the vehicle. This can be done e.g. after first release of the brake after start of travel. For safety reasons, these steps can also be performed several times during continued operation of the vehicle to check whether the coordinates found now as before are coincident with the related positions. Thus, the actuator reference position can be assumed, and the actuator actual position can be compared with the actuator reference coordinate. A correction may become necessary due to wrong sensor pulses or a faulty transmission of the pulses (e.g. too many or too few pulses).
The proper monitoring of the motion of the actuator can be effected in that an actuator actual coordinate is continuously determined as a function of the motion of the actuator. The actuator actual coordinate and/or the actuator motion direction can be determined or detected by using an incremental position measuring system of relative measurement. This determination can take place in that the counter, starting from the actuator reference coordinate, counts up or down the pulses of a Hall sensor installed at an appropriate location, depending on the direction of motion of the actuator. The direction of motion can be detected by using an appropriately installed second Hall sensor.
The current actuator actual coordinate can be compared with the permitted release end coordinate and the first permitted brake application end coordinate, possibly, by taking into account the direction of motion of the actuator to find out whether the actuator actual coordinate lies outside the first permitted actuator motion range. If this is the case, a corresponding signal will be issued.
The signal can be used to send an alarm to the driver. When the actuator actual coordinate exceeds the first permitted actuator motion range in the direction of release, this can be regarded as a malfunction of the engine because there might be the risk in this case that the spindle moves against the engine housing. To prevent this action, the actuator release motion can be stopped in addition to the output of the alarm. After the release movement of the actuator is terminated, several subsequent actions are possible such as a new initialization or deactivation of the brake, etc.
When the actuator actual coordinate exceeds the first permitted actuator motion range in the direction of brake application, an alarm can also be sent to the driver indicating, for example, that brake lining replacement is necessary.
Preferably, the permitted release end coordinate and the first permitted brake application end coordinate are established with respect to the maximum possible mechanical actuator motion range, that means, they can be so established that the spindle will not move against the mechanical stop in the direction of release and will not move so far in the direction of brake application that in the extreme case the metal of the brake shoe will bear against the metal of the brake disc or brake drum, if brake linings no longer exist. Besides, corresponding safety distances can be taken into consideration.